from gameboy, snes, gen and nes what are the best running emulators to use for those?
i see options for all of them but no clear cut choice as to which is the best. nes most roms seems to work with the emu ive tried. gen a little worse, gameboys most seem to work, and snes alot seem to have errors. and alot of the hacks dont like to work either

It would have been nice if you told us which emulators you were using rather than just saying "the one I have works this way" but whatever.

NES - NES DS, possibly a modified version if you want to play Punch-Out or want to use button combos for file operations rather than the touchscreen menu.
GB/GBC - Lameboy v0.12
Genesis - jEnesisDS latest version (I think 0.7.4?) - don't complain about the cropped sides of the screen, it won't do you any good.
SNES - SNEmulDS - requires lots of fiddling with the options to get many games playable, so check some compatibility lists to see if any special tricks are known for the games you want to play. Doesn't work with most game hacks, like you said.

Lack of interest, lack of time, or both. Can't really blame them, after the countless hours they spent getting the emulators into the state they're already in.

SNES emulation is probably a bit trickier than it should be because the DS doesn't have the horsepower to emulate it in software at full speed. Instead, the emulators use what's known as "hardware rendering" in which they attempt to map SNES graphics instructions to equivalent instructions for the DS's dedicated 2D hardware. This allows them to draw to the screen really really quickly (less than 5% of the CPU time) but since the graphics hardware isn't exactly the same, there are graphical glitches and layering issues.

The DS only has 66 MHz to work with, remember (the second 33 MHz processor is used for audio decoding). The PSP still struggles to emulate some SNES games at full speed, and that's with 333 MHz and 32/64 MB of RAM to work with. The state of SNEmulDS is a bit disappointing, yes, but it's not entirely surprising why it works the way it does.